1. Field of the Invention
This invention relates to converters whose operating frequency changes with load and/or input voltage and more particularly to a preload for use in such converters which is controlled as a function of operating frequency.
2. Description of the Prior Art
There are many different topologies which can be used in DC to DC converters. One such topology is that disclosed in U.S. Pat. No. 4,415,959 which issued on Nov. 15, 1983 and is entitled "Forward Converter Switching At Zero Current" (hereinafter "the '959 patent"). That topology, which will be referred to hereinafter as "the quasi-resonant topology", is quite attractive in that it generally operates at a relatively high frequency and the switching device which is connected in series with the source can be switched on and off at essentially zero current.
The one drawback to the quasi-resonant topology as with any resonant topology is that the operating frequency decreases with changes in load and/or input voltage. More specifically, as either the load current decreases or the input voltage increases, the operating frequency decreases from its maximum which occurs for full load and low input voltage. Should the operating frequency decrease sufficiently, the converter operation will become audible. That is undesirable. Therefore, it is desirable to limit converter operating frequency so that it does not fall into the audible range for light load and/or high input voltage operating conditions.
One example of a resonant converter topology which includes circuitry to limit the operating frequency at light loads such that it is not within the audible range of frequencies is shown in U.S. Pat. No. 4,587,604 entitled "Power Supply Employing Low Power and High Power Series Resonant Circuits" and which issued on May 6, 1986. That patent is assigned to the same assignee as is the present invention. The supply shown therein is a series resonant converter which includes both low power and high power series resonant circuits. Both circuits operate when the load current is large. The high power circuit is controlled to cease its operation when the load current decreases to a certain level.
Another example of a series resonant converter which includes circuitry to limit the operating frequency at light loads is shown in U.S. Pat. No. 4,691,273 entitled "Series Resonant Converter With Parallel Resonant Circuit" and which issued on Sept. 1, 1987. That patent is assigned jointly to the same assignee as is the present invention and Nippon Telegraph and Telephone Corporation. The supply shown therein includes a parallel resonant circuit located between the clamp diodes and the dominant series resonant capacitors. The parallel resonant circuit has a predetermined resonant frequency which is set to be above the telephone voice band. The circuit limits the operating frequency of the converter for light loads to be no lower than the parallel resonant frequency.
A further example of a series resonant converter which includes circuitry to limit the operating frequency at light loads is shown in U.S. Pat. No. 4,727,469 entitled "Control For A Series Resonant Power Converter" and which issued on Feb. 23, 1988. That patent is also assigned to the same assignee as is the present invention. The supply shown therein includes a low power control as part of the control circuit. When the loading conditions at the output of the converter are at or below a predetermined level, the low power control circuit causes the converter to operate at a frequency which is either above or below the telephone voice band.